A method to estimate the boson mass and to optimise sensitivity to helicity correlations of τ + τ − final states

In proton-proton collisions at LHC energies, Z 0 and low mass Higgs bosons would be produced with high and predominantly longitudinal boost with respect to the beam axis. This note describes a new analysis tool devised to handle this situation in cases when such bosons decay to a pair of τ -leptons. The tool reconstructs the rest frame of the τ + τ − pair by finding the boost that minimises the acollinearity between the visible τ decay products. In most cases this gives a reasonable approximation to the rest frame of the decaying boson. It is shown how the reconstructed rest frame allows for a new method of mass estimation. Also a considerable gain in sensitivity to helicity correlations is obtained by analysing the τ -jets in the reconstructed frame instead of using the laboratory momenta and energies, particularly when both τ -leptons decay hadronically.